Most of the documents currently in use, such as bank notes, credit cards, identity cards, railroad tickets, airline tickets, checks and the like, can be forged by modern reproduction methods with no great difficulty. Many proposals have been made with the aim to provide such documents with authenticating features, which increase the expense of successfully forging the document, and therefore increase its security, that is, lower the probability that it may be successfully forged. A relatively high degree of security from forging is attained, if authenticating features in the form of spatial structures are embedded in the document, which bend or diffract incident light in a characteristic manner. Such spatial structure, for example, holographically manufactured structures, phase diffraction gratings and kinoforms, can only be manufactured at a high technical cost. As a result of measurable aspects of spatial intensity distribution of light scattered from such documents, automatically operating test devices may determine the genuineness of such documents.
There is known a method of determining the genuineness of a document or security blank from German Patent DE2538875, which includes a light-redirecting spatial structure establishing at least one authenticating feature. The method includes the steps of illuminating the spatial structure, forming from light scattered from the spatial structure two narrow bundles of light rays, converting the two bundles of light rays into electric signals, determining a characteristic measurement value for the spatial structure, and forming "yes" or "no" outputs in dependence of the characteristic measurement value. The document includes a phase diffraction grating, which diffracts or bends the energy obtained from a light source in a predetermined ratio along individual specified directions. When examining the document and determining whether or not it is genuine, it is considered genuine when the diffractions of light energy distributed along certain directions have a predetermined ratio to one another.
From German Patent DE2814890 it is also known to determine the genuineness of a document which has a phase diffraction grating embedded therein, by comparing the intensity of light diffracted along a certain direction with the intensity of diffusely scattered light.
It is also known to embed a security thread in bank notes which has the form of a flat metal strip or plastic strip and a rectangular cross-section. Such security threads which are easily visible, and can also be easily felt, permit a simple and rapid examination relating to the genuineness of the document. Embedding of the security thread in a paper layer or plastic layer requires, however, a costly process, which is difficult for a forger to reproduce. To further decrease the risk of forging, and thus enhance the security of the document, and to automatically establish the presence and genuineness of the security thread by a machine, it is known from German Pat. No. DE2205428 to provide the security thread with microscopically small openings, which may, for example, represent a code pattern, which can then be read out with the aid of light beams or particle beams.